A heat conductive resin film layer that transfers heat from a heat-generating member of an electrical or electronic instrument or the like to a heat-dissipating member of the instrument or the like is conventionally required to have high heat conductivity and insulating properties, and heat conductive sheets obtained by dispersing an inorganic filler in a thermosetting resin have been widely used to satisfy such demands. Here, hexagonal boron nitride (h-BN) having high heat conductivity and further an insulating property has been used as the inorganic filler.
The crystalline structure of hexagonal boron nitride is a layer structure similar to that of graphite, and the particles of hexagonal boron nitride are of scaly shapes. As illustrated in FIG. 10, the scaly boron nitrides each have such an anisotropic heat conductivity high in the length direction and low in the breadth direction. It is said that the heat conductivity in the length direction is several times to several tens of times as high as that in the breadth direction. In FIG. 10, the direction of the arrows represent the direction of heat conduction, and the thickness of the arrows represent the magnitude of the heat conduction. In view of the foregoing, the development of a heat conductive sheet having drastically improved heat conductivity in its thickness direction has been expected, which is attained by orienting the scaly boron nitrides dispersed in a thermosetting resin so that the scaly boron nitrides may be brought into such states as to stand upright in the sheet, that is, their length directions may coincide with the thickness direction.
However, in known molding methods for molding sheet shapes, such as press molding methods, injection molding methods, extrusion molding methods, calender molding methods, roll molding methods, or doctor blade molding methods, there is a tendency for the scaly boron nitrides in the thermosetting resin to be oriented in a state where the scaly boron nitrides topple over in the sheet, that is, such a state in which, as illustrated in FIG. 11, the length direction of the scaly boron nitride 5 may coincide with the sheet surface direction owing to pressure or flow at the time of molding. Accordingly, although a heat conductive sheet thus obtained may have excellent heat conductivity in its surface direction, there is a problem that the sheet cannot sufficiently exert its excellent heat conductivity when the sheet is used with the thickness direction as the heat conduction path.
In view of the foregoing, several methods for having the length directions of the scaly boron nitrides dispersed in the thermosetting resin coincide with the sheet thickness direction, that is, orienting the length directions of the scaly boron nitrides so that they stand upright in the sheet thickness direction have been proposed.
For example, in Patent Document 1, a heat conductive sheet, in which the length directions of scaly boron nitrides are oriented upright in its thickness direction, has been obtained by slicing in its thickness direction a sheet in which the length directions of the scaly boron nitrides are oriented so as to coincide with its surface direction.
In Patent Document 2, a heat conductive sheet in which the length directions of scaly boron nitrides are oriented upright in its thickness direction has been obtained by vertically cutting a laminated product obtained by winding a sheet in which the length directions of the scaly boron nitrides are oriented so as to coincide with its surface direction.
In Patent Document 3, a heat conductive sheet, in which the length directions of scaly boron nitrides are oriented parallel to its thickness direction by a magnetic field, has been obtained by applying the magnetic field in the thickness direction of the sheet before curing a polymer composition containing the scaly boron nitrides.
In Patent Document 4, a heat conductive sheet, in which the length directions of scaly boron nitrides are prevented from being oriented in its surface direction, has been obtained by shaking a composition containing organopolysiloxane and the scaly boron nitrides with an ultrasonic shaker before being cured.
Patent Document 5 proposes a heat conductive sheet in which an inorganic filler containing the secondary aggregation particles of scaly boron nitrides is blended.
Patent Document 1: JP 06-38460 B    Patent Document 2: JP 3568401 B    Patent Document 3: JP 2002-80617 A    Patent Document 4: JP 06-12643 B    Patent Document 5: JP 2003-60134 A